In this type of cable, an inner conducting core made, for example, of twisted strands of a conducting metal is surrounded along the length of the cable by a cylindrical layer of paper that is impregnated with an oil. This layer is in turn surrounded along the length of the cable by a lead (or other conducting material) jacket or sleeve. The impregnated paper layer acts as a dielectric that insulates the inner conductor from the outer (conductive) jacket.
PILC's are rarely manufactured nowadays, but many tens of thousands of metres of the cable type remain in service around the world. Therefore there remains a need for connecting PILC's together, and for connecting PILC's to other types of cable.
One characteristic of a PILC is that if migration of the oil occurs, the dielectric effect of the impregnated paper diminishes dramatically as the paper dries out. Generally this migration effect does not occur over the length of cable that lies away from the cable ends, but when it is required to splice an end of a PILC a potential problem arises in that there exist leakage paths for the oil.
In addition, oil may also affect the mechanical performance of many polymers and so must be prevented from coming into contact with any such polymers, for instance, the polymeric insulation of modern power cables or other polymeric parts in a cable joint, in particular polymeric parts that have conductive properties.
Prior art designs of cable joint for use with PILC's have sought to close off such leakage paths. This aim may be realized through the use of a rigid casing, of the kind described in GB-A-1 485 613, that encloses a joint assembly including various seals. The objective of including the seals is to prevent deleterious migration of the oil from the paper layer.
The arrangement of GB-A-1 485 613 however is complicated. Aside from the fact that this makes the joint expensive to manufacture, assembly of the joint is a lengthy process, the steps of which must be completed in the correct order in order to assure leak-proofing of the joint. If the joint is assembled in a “field” situation by an inexperienced fitter therefore, there is a danger of the steps not being completed correctly or in the correct order, such that the joint fails in service. Similarly if even one of the many components of the GB-A-1 485 613 joint becomes lost or damaged the integrity of the joint is compromised.
Yet a further problem with the joint of GB-A-1 485 613 is that it does not seek to address the problems that can arise when the heating effect of electrical resistance in the cable of a PILC causes expansion of the oil of the impregnated paper.
Under such circumstances the pressure of oil within a joint can rise sufficiently that the oil is forced to leak away, thereby reducing the dielectric effect as aforesaid.
High oil pressures can also arise when the cable containing the joint lies, for example, on a hillside. The hydraulic head of oil above the joint can then be adequate to promote the above-described migration of oil.
Known cable joints may generally comprise an oil barrier tube which may be in the form of, for example, a rigid casing which is used in conjunction with a sealing mastic. Under fault conditions, the cable temperature can increase significantly leading to a large increase in oil pressure. A problem with known cable joints of this type is that they are not able to withstand such increases in oil pressure.
Experiments have shown that sealing mastic deforms when exposed to high air pressures. Under such conditions, voids having a generally “teardrop” shape, begin to form as the air pressure forces a path through the mastic. The voids propagate, eventually forming pathways through the mastic.
FIG. 1 illustrates schematically the approximate shape of voids formed in mastic when the mastic is subjected to high air pressure. The higher the pressure the more quickly the voids will form. At a pressure of 5 bar, voids may form very quickly, whereas at a pressure of 3 bar, voids are unlikely to form, or may form very slowly.
It is understood by those skilled in the art that a similar deformation will take place when mastic is subjected to high oil pressure. Sufficiently high oil pressure for void formation is likely to occur under heavy loading or fault conditions. Under such conditions the voids formed are likely to be relatively wider than those shown in FIG. 1.
In known cable joints comprising an oil barrier tube used in conjunction with a sealing mastic, voids formed in the mastic may propagate through the mastic when the oil pressure within a joint rises significantly. As the voids propagate, they form pathways providing means for oil to pass through the mastic and hence leak from the cable.